Implementing new input types email, url and tel:

- adding checkboxes to enable the user to define a text_field as an email, url or phone number.
- adding variable $field_type with default value "text".
- use the new checkboxes to change the $field_type variable to one of the new HTML5 input types.

A first step for the enhancement in #286 as a quick solution. It should also be nice, if the new types would be available just as the email field with the checkbox preselected. But I haven't found the functionality to do so.